1. Field of the Invention
The present invention relates to a method of fabricating a ceramic circuit substrate having via hole conductors which has excellent electrical connectivity.
2. Discussion of the Prior Art
Conventionally, ceramic circuit substrates are fabricated by printing conductive patterns on the ceramic greensheets, laminating the ceramic greensheets to form a laminating body and thermocompressing, then sintering them.
In the sintering process, the ceramic substrates, because of sintering the ceramic greensheets, shrink in three dimensions by 10 to 20% and, therefore, the ceramic substrates differ in dimensions and positional accuracy of the conductive patterns. This makes it difficult to place ICs and tip capacitors in position on the ceramic circuit substrate.
In an alternative method of fabrication, unsintered ceramic sheets which do not sinter at the sintering temperature of the ceramic greensheets are placed on the outermost layers of the ceramic greensheets. The unsintered ceramic sheets are bonded to the outermost layers by thermocompression. Then, the ceramic greensheets are sintered. In this method, shrinkage of the ceramic substrates in the planar directions is suppressed. The ceramic substrates shrink principally only in the direction of their thickness dimension. In this case, however, the via hole conductors filled into the via holes, which pass through the ceramic substrates, shrink, and consequently the connectivity of the conductors with the inner walls of the via holes deteriorates. More specifically, the ceramic substrates shrink in their thickness direction, but the conductor in the via holes contracts in three dimensions. For this reason, gaps are created between the conductor surfaces in the via holes and the inner walls of the via holes. Therefore, gaps may be formed between the conductors in the via holes and the conductive patterns on the internal layer circuits the patterns being connected with the conductors. As a result, electrical connection may be impaired.
As shown in FIG. 14, internal layer circuits 58 and 59 are formed inside the ceramic circuit substrate which is prepared by the above-described procedures. These internal layer circuits are electrically connected through via holes 90. Each of the via holes 90 has a via hole conductor 5 located therein. The internal layer circuits 58 and 59 are electrically connected together at the side surfaces of the via holes 90.
What we have mentioned above will be described more in detail by using FIG. 15. A method of fabricating the above-described ceramic circuit substrate 9 having internal layer circuits therein is as follows.
First, as shown in FIG. 15, the via holes 90 are punched in ceramic greensheets 77-79 in registration with each other, the ceramic greensheets being used for fabrication of the ceramic substrates 97-99. The via hole conductors 5 filled into the via holes 90. Thereafter, the internal layer circuits 58 and 59 are printed on the surface, respectively, of the ceramic greensheets 78 and 79. Subsequently, unsintered ceramic sheets 61 and 69, which are not capable of sintering at the temperature at which the ceramic greensheets 77-79 are already sintered, are placed on the top and bottom surfaces, respectively, of the ceramic greensheets 77-79 and bonded thereto by thermocompression. Then, these sheets are sintered at the above-described sintering temperature, thus producing the ceramic substrates 97-99. Then, the unsintered ceramic sheets 61 and 69 are removed.
In the above-described prior art method of fabrication, when the ceramic greensheets are sintered, via holes 90 curve as shown in FIG. 16, which results in gaps 8 in via holes 90. Therefore, via holes 90 and the internal layer circuits 58 and 59 may break. Hence, the prior art configuration lacks reliable electrical connections.
It is postulated that via holes 90 become curved for the following reasons.
During sintering, shown as in FIG. 16, the ceramic greensheets 77-79 shrink, since the top of the ceramic greensheet 77 and the bottom of the ceramic greensheet 79 are constrained by the unsintered ceramic sheets 61 and 69. Therefore, the ceramic greensheets 77-79 shrink in the vertical direction. However, the side surfaces of the ceramic greensheets are in a free condition. Consequently, the portions around the centers of the via holes 90 shrink to a significant extent and thereby curve and deform. The curvature and the deformation increase as the thickness of the entire compressed body of ceramic greensheets 77-79 increases. When the ceramic circuit substrate is used at locations where high temperatures alternate with low temperatures over a long period of time because of the cycles of thermal expansion and contraction of the via hole conductors and the ceramic substrates, breakage of connections might occur between the via holes and the internal layer circuits.